Looking at the file's history revealed this commit that initially raised the required version: 3bdf9038213a4a32f27537c43c7d82602cfae5b2

The relevant commit does not affect the code at all and only raises the dependency. Perhaps we can check if we can loosen the required version to support both ~2.8 and ~3.0 like this:

```
"guzzle/parser": "~2.8|~3.0"
```

(This would be in line with the suggested changes in https://github.com/reactphp/http-client/pull/2#discussion-diff-13262490)